As if that news weren’t alarming enough, the study says an additional 100 million people in cities could be water short because of the impacts of climate change…while up to 3 billion could be in water shortage at least one month out of the year.

Rob McDonald, lead author of the study, says there’s still time to take action against these shortages. I asked him to go deeper into the numbers and tell us which cities might be the biggest water losers unless something changes.

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Q: Have you ever tried to live on 2/3 of a bathtub of water a day? What’s it like?

A: After this paper came out, I tracked my family’s use of water for a week — we average out to around 130 liters/person/day, so a bit above the threshold we used in the paper.

Doing that did help me identify some quick ways I could save water. I have an old house, and I probably just need to install some low-flow faucet aerators. Right now some of my faucets don’t have any aerators, so it turns out I’m wasting a lot of water! I also have old clothes-washing and dish-washing machines. I’m too cheap to change those right away, but once they break I’m now motivated to replace them with low-flow machines. If I did that, we’d be below 2/3 of a bathtub of water a day.

Q: The first response that often comes up to studies involving population is “Isn’t this just a matter of overpopulation? People should stop having babies, and that will solve the problem.” How do you respond to that line of thinking?

A: It’s just a really simplistic way to look at a very complex issue, and kind of a cruel way. Having children is one of the most important, life-defining experiences for most people, and many people would argue it’s a basic human right.

You have to look at population growth in the full context of what’s called the demographic transition. In very poor countries, the birth rate is high, but so is the death rate. As countries economically develop, there is a tendency for the death rate to fall first, as very basic sanitation measures are implemented. Then, at slightly higher economic levels of development, the birth rate falls too.

If you look at where some European countries are right now, they are actually losing population, the birth rate has fallen so much. So what is happening in the developing countries is that they are simply earlier along in the demographic transition than the US or Europe. At one point, we too had very rapid population growth — we just got past that stage.

Another thing to keep in mind is that there is significant demographic momentum toward increasing population in developing countries simply because there are so many young people in these countries. Even if every young person alive today in developing countries limited themselves to 1 child/family, overall population would still go up.

Finally, a lot of urban growth has nothing to do with the population dynamics within a city- it’s a result of urban to rural migration, millions of people moving to cities in search of a better life. The U.S. and Europe went through this same phase too, just 50 to 100 years ago.

Q: Seasonal water shortages — the numbers that might be affected are astronomical. But is this condition as bad as it sounds? I wouldn’t want to go through it, but just how damaging to a person is a month out of every year without adequate water supplies?

A: What we call seasonal water shortages in our paper aren’t that uncommon. Many cities in the western U.S. have seasonal scarcity. But cities traditionally get around this by having dams for storing water, or from diversifying their water supplies so they can find water in those dry months. So the big number of potential future seasonal shortages that we calculate in our models show that there are lots of people — billions of people, in fact — who are in cities that will actively have to search for new solutions to seasonal scarcity, either because the city’s population has grown or because climate change has affected the seasonal distribution of rainfall.

Bottom line: Don’t think of those high numbers as a forecast of doom. They are a call to action.

Q: We hear about different scenarios for different climate change projections in other studies. What about this study?

A: We looked at several climate change projections as well. And the exact results do vary among the scenarios. Basically, for some regions of the world, all the different climate change projects are really consistent, and we can make reasonable predictions about what will happen in those places.

For instance, North Africa and the Levant look pretty likely to get less precipitation with climate change, and be drier. Then there are places where the different climate change projections don’t agree, and there’s a lot less certainty about what will happen. For instance, it’s not clear whether Australia will see a decrease in precipitation or an increase.

People can use the Climate Wizard tool (which The Nature Conservancy helped create) to check out the different climate change projections themselves, for their county or city.

Q: So you calculated for water that was available to cities right around them, and then you calculated for water available to a city 100 kilometers around it. Given that cities are going to try to tap water from the surrounding countryside, does that second set of numbers give us a more accurate picture of how many people are actually going to be short of water?

A: Cities go as far away as they need to to get enough water. That’s why we calculated the amount of water at various distances from cities, to figure out how far away cities might need to go to get water. That correlates with how much infrastructure cities need to build, or how much money they need to spend.

Note also that our study was just looking at sustainable flows of surface and groundwater. There are other ways to get water, of course. Some cities will pay to desalinate ocean. Some cities will build canals to get water from hundreds or thousands of miles away. Both those solutions are really expensive, though. The most common solution, when cities can get away with it, is to unsustainably pump groundwater from an underground aquifer. But that only works for a few decades, until the aquifer is pumped dry.

Q: How hopeful are you that we are going to be able to head off these kind of massive water shortages? What will it take to do so — and do we have the mechanisms in place?

A: I’m pretty hopeful about what you might call middle-income countries. That includes China, India, Mexico — countries that are experiencing urban growth but have some financial resources are generally doing a good job planning for how to bring water to their urban residents. Sometimes these cities are too reliant on grey infrastructure (dams and canals), and ignore the effects of their actions on freshwater ecosystems, but at least they are thinking about the problem. It’s an opportunity for ecologists, really, to try to make the plans of these cities more compatible with freshwater biodiversity.

What I really worry about are cities in some very poor countries, places with very few resources. I think for them, they will need outside help funding the necessary infrastructure or land-use changes.

I think there’s a role for the international community to fund some of this infrastructure creation. Picture this: Instead of each country having to separately borrow funds to build urban water infrastructure, there’s a common pool of money that could finance urban water planning and management in all developing country cities. To some extent, that exists now with some of the major development banks. But there’s a potential I think for that kind of thing to be substantially scaled up.